Paper

A common misconception for aerobic digestion is that the process must be aerated 100% of the retention time. In redefining this outlook, it has been found that cycling between mixing and mixing while aerating proves to be more effective and energy efficient. By having the ability to turn the air off and on, the system creates aerobic and anoxic phases allowing for further nutrient reduction, improved dewaterability, and increased volatile sludge destruction. This supports endogenous respiration and nitrification. Many different technologies exist for aerobic digestion, and often multiple types of equipment must be combined to achieve the cyclical process. Fortunately, there are systems that can perform decoupled mixing and aerating with only one device.

Defining aerobic digestion

Aerobic digestion is a wastewater treatment process used to treat waste activated sludge or a mixture of sludges. Typical applications produce a sludge able to meet requirements for Class B biosolids, and retention times usually range from 40 to 60 days. Sludge destruction is primarily a direct function of sludge age and temperature shown in Figure 1 (Metcalf & Eddy, 2014).

Aerobic digesters can achieve thermophilic conditions which provide more rapid biochemical reactions and reduce the retention time 20 to 40 days typically. Thermophilic conditions can provide greater reduction in bacteria and viruses, meet Class A biosolid requirements when operating at 55º C or higher, and lower energy requirements than conventional aerobic digestion. Although advantageous to aerobic digestion, this article will not discuss thermophilic conditions further and focus only on the traditional aerobic digestion process.

Aerobic digestion is the degradation of the organic sludge in the presence of oxygen. Oxygen is introduced in the basin or tank to allow the micro-organisms in the sludge to convert the organic material to carbon dioxide and water, and the ammonia and amino species to nitrogen. Biochemical changes in an aerobic digester follow the subsequent equations:

Biomass destruction: Biomass + Oxygen → Carbon Dioxide + Water + Ammonium Bicarbonate

Nitrification: Ammonia + Oxygen → Nitrate + Hydrogen + Water 

Overall Equation: Biomass + Oxygen → Carbon Dioxide + Water + Nitric Acid

Denitrification: Biomass + Nitrate → Carbon Dioxide + Nitrogen + Ammonia + Hydroxide

Complete Process: Biomass + Oxygen → Carbon Dioxide + Nitrogen + Water 

Aerobic digestion is similar to the conventional activated sludge process but has longer retention times without a raw wastewater feed i.e. nourishment for the micro-organisms. When there is no new supply of organics for the micro-organisms, they die and become nourishment for other bacteria in the tank reducing the sludge organic solids concentration. This process is known as endogenous respiration. Aerobic digestion also has the ability to nitrify under certain conditions. Typical operations are controlled by pH; however, other parameters can be used to control the process.

Current technologies

Since both air and mixing are required for the aerobic digestion process, typical equipment may include coarse bubble aeration, jet aeration, surface aeration, and fine bubble aeration. To be able to do both, mixing and mixing while aerating, often times, multiple devices must be used.

Technologies exist with the capabilities to perform mixing and mixing while aerating, eliminating the need for two different systems. For the ability to cycle between mixing and mixing while aerating, the INVENT HYPERCLASSIC^®-Mixing and Aeration System has proven suitable for the application. The system provides the ability to mechanically mix and aerate for aerobic digesters. The air can be turned off while still mixing, creating anoxic conditions so that denitrification can occur. This system provides optimal control of a process that was once mostly inflexible and has significant advantages over other systems such as e.g. surface aerators (see Fig. 2).

The alpha value

With 2% or higher mixed liquor suspended solids (MLSS), mechanical mixing is required due to the viscosity of the liquid because air alone will not provide sufficient mixing. As the percent solids increase, there is a correlation with a decrease in the alpha value. The alpha value is the interference to oxygen transfer efficiency, and data indicates that there is strong correlation between fluid viscosity and oxygen transfer efficiency. In the article “Digester Aeration Design at High Solids Concentration”, research has shown that with fine bubble diffusers, the alpha value falls below 0.1 around 3% MLSS (Schoenenberger, Shaw, Redmon, 2003).

When the alpha value falls below 0.1, this can lead to unwanted anaerobic conditions causing odor and foaming, the HYPERCLASSIC^®-Mixing and Aeration System provides higher alpha values as the mechanical nature of the device pushes oxygen deeper into the sludge flocs.  The alpha value for solids ranging up to 5% can be around 0.27 for this mixing and aeration system. This results from the mechanical mixing, which allows for the distribution of air throughout the tank, and system providing a small to medium bubble for better oxygen transfer.

Cycling between mixing and mixing while aerating

Unlike the over-aeration from conventional continuous aerated mixing, a decoupled mixing and aeration system provides control over the entire aerobic digestion process through oxygen supply and/or mixing. Different parameters must be established to control the aerobic digestion process, both time and ORP (oxidation reduction potential) can be used to control the cycling between mixing and mixing while aerating alongside pH.

Since solids reduction is typically the main goal of aerobic digestion, changes in pH dictate the ability to destruct the biomass. When pH drops, alkalinity addition becomes necessary to combat poor dewaterability of the sludge. When the air is cycled off, alkalinity can be restored through nitrate destruction (denitrification). This off/on air cycle can now decrease the need for chemicals that are necessary for continuously aerated systems.

Nitrification can also be a main goal of aerobic digestion and is advantageous when ammonia in the digestate is of concern at the head of a plant. Starting with mixing and aerating, the biomass in the system destructs creating carbon dioxide, water, and ammonium bicarbonate. With air still being introduced into the system, nitrification can now happen converting ammonium to nitrate, hydrogen, and water. If air is always on, this continued process can drop pH and consume alkalinity. Now, introducing long periods of mixing only, the aerobic digester essentially becomes anoxic/anaerobic with zero oxygen throughout the entire basin. This cycle begins the denitrification process where nitrate combines with the hydrogen from the nitrification to form nitrogen and water, and in the presence of biomass, also creates carbon dioxide and ammonia. Denitrification restores pH by returning alkalinity back into the system. With the pH restored, the aerobic digesters can turn air on beginning the cycle over again. At the end of the process, the digestate now has up to 20% less ammonia returning back to the head of the plant.

The HYPERCLASSIC^®-Mixing and Aeration System: An overview

By enabling the operator to have more control over the aerobic digestion process, plants have the ability to reduce cost, reduce chemical needs, and increase energy savings with cyclical aerobic digestion. With the HYPERCLASSIC^®-Mixing and Aeration System, mixing energy is dissipated at the point of air introduction creating a surface area of constantly renewed air bubbles when air is turned on while maintaining sufficient mixing when air is turned off. This creates the ideal environment for cyclical aerobic digestion.

The system itself has a hyperboloid mixer body that is non-clogging and has integrated transport ribs for optimal fluid acceleration.  There is consistent backpressure and aeration efficiency that allows for no deterioration of efficiency over time providing the ability to turn the air on and off whenever necessary. Even with high MLSS, the Mixing and Aeration System is robust, and also easy to maintain and operate. The system only requires above water maintenance on the dry mounted gear motor, and inspection on the bottom guide mounted to the floor of the tank every few years. The system is able to operate at varying water levels, with the ability to drain the tank or basin completely with no harm to the system when decanting or during the removal of the sludge. This makes the decanting process streamlined and easy due to the sludge being continuously mixed even as the water level is decreasing.

Intensive mixing is important to prevent settling on the bottom of the tank, and with poor mixing, oxygen gradients can occur. Since the mixer body of the HYPERCLASSIC^®-Mixing and Aeration System is installed close to the bottom of the tank, oxygen gradients are prevented and a homogeneous sludge is created. Air is generated from the sparger ring underneath the mixer body. There the air escapes and meets the uniquely shaped underside of the mixer body, which is equipped with dispersing tunnels and special shear fins. As the mixer body rotates, the air in the dispersing tunnels is mixed intensively with the wastewater which creates a dissolution of coarse to fine bubbles by the shear fins. The main flow then transports these bubbles radially outwards and distributes them throughout the whole tank.

Real world applications

With any technology, real-world applications and data show if a system works. The HYPERCLASSIC^®-Mixing and Aeration System has been installed in digesters around the world proving to be a suitable application for cyclical aerobic digestion. In the United States, there have been multiple aerobic digestion installations with process success. The oldest running aerobic digesters with these Mixing and Aeration Systems in the United States have been running since 2006 at the Jacksonville Beach WWTP (see Fig. 4).

The aerobic digesters at Jacksonville Beach treat to 3% solids in three circular tanks and provide a consistent quality feed to their sludge dewatering process. The sludge treated continues to meet Class B standards efficiently, and the operators are very satisfied with their choice and the resulting wastewater treatment. Another installation is across the country in Meridian, CO (see Fig. 5).

They have an aerobic digestion lagoon system completed in 2020 with two HYPERCLASSIC^®-Mixing and Aeration Systems. The operator has the ability to run the them at varying water depths with ease, and there is little to no smell when the standing directly over the basin. Sludge is treated to 2.5% solids at half the capacity of the previous design. This has helped decrease energy costs and increase process performance.

Summary

The cyclical process used in aerobic digestion has proven successful in both solids reduction and nitrification. The INVENT HYPERCLASSIC^®-Mixing and Aeration System has proven suitable through real installations even at high percent solids. With the ability to cycle the air on and off while mixing, the system provides flexibility for the operator and the operation. The systems robustness and mechanical reliability make it a top choice for aerobic digesters.

Literature

Metcalf & Eddy Inc., Tchobanoglous, G., Burton, F. L., Tsuchihashi, R., & Stensel, H. D. (2014). Wastewater Engineering: Treatment and Resource Recovery (5th ed.). McGraw-Hill Professional. 2014.

Schoenenberger, M., Shaw, J., & Redmon, D. (2003). Digester Aeration Design at High Solids Concentrations. Presentation at the 37^th Annual Wisconsin Operators Association Conference.

In water treatment, the process of flocculation can play a pivotal role in meeting required water quality standards. Today, water providers face increasing concerns over water regulation, pollution, and budgets. Retrofitting existing water treatment facilities with advanced flocculation technology is fast becoming a proven solution to meet evolving regulatory requirements and community needs at a reasonable cost. This paper explores the significance of flocculation in water treatment; diving into principles, applications, and current technologies. Analyzing recent retrofits and case studies provides insights into the practical implementation of upgrading existing water treatment infrastructure.

2. Defining Flocculation

Flocculation constitutes suspension and gentle collision of particles in water to form larger aggregates known as floc all while avoiding sedimentation. Floc must achieve a specific size to facilitate removal during sedimentation of the water treatment process. The effectiveness of flocculation largely relies on the introduction of chemical agents, such as cationic polymers, during the coagulation stage. These chemicals, typically positively charged, serve to neutralize the negatively charged particles in the water. This neutralization promotes the agglomeration of particles into larger floc.

Successful flocculation is a delicate balance of adequate mixing to encourage the collision and aggregation of floc, while simultaneously minimizing shear forces to prevent disintegration. This equilibrium ensures the formation of stable floc that settle out effectively, optimizing the overall efficiency of the treatment process.

3. Current technologies

Coagulation, flocculation and sedimentation are interconnected processes which rely on one another. While this paper focuses on flocculation retrofits, it’s important to acknowledge the advancements in using improved coagulants and flocculants engineered for better performance and cost-effectiveness. These chemicals are selected based on their efficiency in neutralizing suspended particles and promoting floc formation. While this paper doesn’t dive deeper into this topic, choosing the best coagulant for different source water qualities remains a critical component in water treatment.

When retrofitting current technologies into existing systems, basin design, flows, transition areas, and inlet/outlet locations are a few important factors that must be carefully examined. By considering the overall hydraulics and flow velocities of a current system, the best technology can be selected for replacement of old methods, and improvements can be made to optimize the flocculation process.

Presently, a wide variety of flocculator methods are installed in the U.S. and abroad. Common types of flocculators include: hydraulic baffle walls, vertical and horizontal paddle wheels, bladed impellers, walking beams, and hyperbolic impellers.

Several factors must be considered to determine the ideal equipment for the treatment plant. Major criteria for evaluation include maintenance requirements, adjustable velocity gradients, power usage, energy transfer, redundancy, and floc quality:

• Maintenance: Paddle wheels and walking beams have parts, such as submerged and bottom bearings located below the water surface, which require basin drainage for maintenance. In contrast, all maintenance for bladed impeller and hyperbolic flocculators is performed above the water surface. Hydraulic baffle flocculators typically require the lowest maintenance, as they have no mechanical parts. However, periodically the entire train must be taken offline and drained for cleaning sediment buildup.

• Adjustable Velocity Gradient: Hyperboloid and bladed impeller flocculators offer the highest level of treatment flexibility, by adjusting the rotational velocity to optimize the velocity gradient. Velocity gradients with vertical and horizontal paddle wheels and walking beams are typically set during design by adjusting the number of paddles. The rotational speed may be adjusted if the units are installed with variable frequency drives. However, speed changes can lead to excessive settling in the floc basins. Hydraulic baffles offer little or no process flexibility as they are designed for one flowrate without adjustment. By being able to adjust the velocity gradient, an operator has the ability to adjust the flocculators, seasonally for temperature, flows, or when treatment chemicals are changed.

• Power Usage: Except for hydraulic baffle flocculators, all other types utilize gearboxes and motors, indicating similar power consumption levels due to velocity gradient requirements per stage. Due to a higher pumping capacity, hyperboloid flocculators may be operated at lower speeds than dictated by velocity gradient alone, providing some energy savings.

• Redundancy: Bladed impeller, vertical paddle wheels, and hyperboloid flocculators offer the highest redundancy if a unit is down. Horizontal paddle wheels have the least amount of redundancy, as multiple units typically operate on one drive.

• Energy Transfer: Shear forces can impact floc quality when energy transfer between the flocculator and the floc particles is not taken into consideration. The hyperboloid flocculator has equal pressure distribution over the shape of the mixer body which acts to minimize shear forces. The acceleration path of the particles is very long, indicating a more gradual transfer of mixing energy to the floc particles. With bladed impellers, the energy from the mixer is transferred over a much shorter acceleration path, meaning higher shear forces. Additionally, pressure developed during floc acceleration is spread only over the surface area of 3 or 4 blades which can increase shearing as well.

• Floc Quality: Properly operated flocculators of all types are known to produce settleable floc. Horizontal Paddle wheels, although an older technology with more maintenance and higher levels of sedimentation, are also recognized for producing good settling floc. Hyperboloid flocculators have an advantage balancing appropriate mixing energy with low shear forces due to the shape and flow patterns.

Computational Fluid Dynamics (CFD) modeling can be a beneficial tool for the design and retrofit of flocculators. CFD enables precise optimization of mixing parameters, further enhancing flocculation efficiency. It aids in identifying short circuits, flow issues, and understanding of the influence of baffles and openings throughout the flocculation stages. By understanding flow, basin design, and overall hydraulics, areas of high shear can be avoided. CFD modeling can even help select the most suitable equipment for flocculation by modeling the different flocculator technologies under the same basin hydraulics and design.

4. Floc Properties

Understanding floc properties is crucial for optimizing flocculation. Floc, formed through the aggregation of suspended particles, exhibit a range of characteristics that influence their behavior and settling dynamics within the treatment system.

One key property of floc is size distribution, which directly impacts settling rates and sedimentation efficiency. Larger floc generally settle more rapidly than smaller ones, aiding in their removal from the water during the sedimentation process. Other factors which significantly affect settling behavior include structure, density, porosity and surface area of the floc. Additionally, floc with a loose, open structure tend to settle more slowly than those with a denser, compact structure. Internal porosity and surface area influence adsorption capacity and interaction with contaminants, further affecting treatment efficiency.

Required velocity gradients and mixing intensities also play an important role in the formation of floc. Velocity gradient is defined as the square root of the ratio of power loss by shear per unit volume of fluid to the viscosity of the fluid. Although it represents the energy level per volume unit, this parameter is also used to predict the intensity of shear and represents the intensity of shear applied to the fluid, which influences the flocculation dynamics by affecting the rate of particle collisions and floc formation.

G = Velocity Gradient or Mixing Intensity (s^¯¹)
P = Power (kW)
V = Tank Volume (m^³)
µ = Viscosity (Pa*s)

While the velocity gradient quantifies the amount of mixing energy imparted to the fluid, it does not fully describe the manner in which this energy is introduced or distributed within the flocculation basin. Understanding the distribution of shear energy and its impact on floc formation requires a more nuanced approach. Specifically, assessing how power is introduced and distributed in the basin. Considering factors such as flow patterns, turbulence, and the geometry of the mixing equipment, this can provide a deeper insight into the flocculator’s performance.

For example, energy dissipation can vary significantly depending on whether the mixing system generates uniform or localized shear, since it affects how efficiently the energy is transferred to the floc particles. By analyzing the spatial distribution of shear and energy within the flocculator, one can better understand how these factors influence floc size, structure, and settling characteristics. Therefore, integrating the concept of energy distribution with the velocity gradient offers a more comprehensive view of flocculator performance and allows for optimization of the flocculation process.

Understanding the properties of floc, including size distribution, structure, density, and velocity gradient, is
essential for designing, operating and retrofitting efficient flocculation systems. By carefully assessing these
parameters and integrating innovative technologies such as hyperboloid flocculators, treatment plants can
achieve superior removal of suspended solids and contaminants.

5. The Hyperboloid Flocculator

6. Real World Applications

Built new in 2015 to replace an existing water treatment plant, the City of Annapolis, Maryland installed hyperboloid flocculators replacing old bladed impellers from the existing plant. Annapolis reported several significant improvements from the previous system which were attributed to the new equipment. The lead
operator highlighted increased treatment capacity in a smaller footprint, particularly praising the hyperboloid flocculators. The City now uses significantly less coagulant for water treatment; approximately 30% less alum resulting in cost savings. The operator also reported that less lime is needed to neutralize the acidifying affect of alum as well. Treated water quality improved with iron levels decreasing from 1.0 ppm in the old system to 0.5 ppm in the new system, despite
reduced coagulant usage. The City of Annapolis noted the reliable performance and minimal maintenance requirements of the new equipment. Sediment accumulation in tanks is minimal and is accredited to the flow patterns generated by the hyperboloid flocculator confirming prior CFD modeling. CFD modeling was performed on four different load cases that considered minimum, average, and maximum flow rates with different rotational speeds. The modeling confirmed that the design would handle all flow rates with the proposed basin layout. The CFD modeling also concluded that even at lower rotational speeds, there would be reliable mixing through the stages, with no areas of high shear.

In 2019, the City of Bellevue, Ohio’s Water Filtration Plant operating since 1935, replaced old horizontal paddle flocculators with hyperboloid flocculators. Bellevue Operations indicated several notable changes and positive impacts in the survey. Bellevue’s respondents noted enhanced first-stage treatment process while operating more quietly than past installed equipment. While they did not observe a reduction in coagulant dosage, they do now have the ability to feed powdered activated carbon (PAC) at the flocculation stage which represents a significant operational enhancement. The City of Bellevue also obtained improved treated water quality, with decreased turbidity in the treated water.

In 2013, Hickory, North Carolina Public Utilities and Systems retrofitted one treatment train of the City of Hickory Water Treatment Plant. Four bladed impeller flocculators were replaced with four hyperboloid flocculators. The survey results of the adoption of hyperboloid flocculators reveal several positive results. Operators reported significantly less settling of solids within the hyperboloid train than the trains with bladed impellers. The design of the basins prevents isolation of the hyperboloid flocculators for dosage adjustment which makes it difficult to determine any changes in chemical dosage. Flow to the hyperboloid flocculators remains consistent with the other trains. While data on treated water quality isn’t tracked for each basin, it’s noted that the flocculation basins with hyperboloid units have less sediment buildup over time compared to the bladed impellers. Although flows are not separated for each train, a recent collection of data indicated lower turbidity in the sedimentation basin following the hyperboloid flocculators at 0.22 NTU average compared to the sedimentation following the bladed flocculators at 0.31 NTU average.

The City of Atlanta, Georgia Chattahoochee Water Treatment Plant, has begun the upgrade of their existing horizontal paddle wheels to the hyperboloid flocculators. The first train to be installed did not initially achieve the expected settled water turbidity. An in-depth study including CFD modeling of the basins with both the hyperboloid flocculators and the old paddle style was conducted to pin-point the issue. It was discovered that the original basin design had significant areas of short circuiting, which was leading to sub-optimal performance. At the manufacturer’s recommendation, baffles were added, indicated by the modeling, to create a serpentine flow and eliminate short-circuiting. This adjustment aimed to improve flow dynamics within the basins containing the hyperboloid flocculators. The correction of the hydraulic short-circuits has led to the upgraded train producing the best settled water turbidity of the plant. While most of the other five trains average 0.55 NTU to 0.90 NTU, the retrofitted train consistently averages 0.15 to 0.50 NTU, even after wet-weather events.

The Houston East Water Purification Plant upgraded their horizontal paddle wheels with hyperboloid flocculators in 2020. Since installing the hyperboloid flocculators, there has been a noticeable improvement in treated water quality, with management and operators expressing high satisfaction. Although there was a switch from Ferric and Lime to PACl, making direct cost comparisons challenging, the new system has significantly reduced the amount of sediment in the tanks, with only about 1 inch of corner sediment compared to the previous 10 feet of sludge buildup. Flow rates have remained unchanged, and no data is available to compare treated water quality between the old and new systems.

Outside of the US in the Czech Republic, the Bzenek Ground Water Treatment Plant replaced horizontal paddle wheels with hyperboloid flocculators to trial in 2007 which led to a complete replacement in 2009. The trial was designed to objectively compare the effectiveness and performance of hyperboloid flocculators versus horizontal paddle wheels. Initially, the same speed was set for the Invent mixers, which was then varied – both increased and decreased. Different speeds were subsequently adjusted for the front and rear mixer pairs. At the start of the trial, the front mixer pairs operated at a higher speed to optimize floc formation in the rear mixers and prevent their disruption. Later, the front pairs were taken out of service while the rear pairs operated, and vice versa. Analysis of the results indicated that maintaining the same speed across all four hyperboloid flocculators created optimal conditions, as a higher shear at the flocculation tank inflow made increased speed of the front pairs unnecessary.

During the trial, power consumption for both flocculators was monitored, allowing for calculations of power requirements, energy consumption, and estimated annual operating costs for the mixers. In conclusion, the results indicate that higher speeds (up to 20 rpm) lead to increased floc destruction, shorter retention times in the flocculation area and sedimentation tank, and higher filtering velocities for total iron, total manganese, and calcium carbonate. The optimal speed for the hyperboloid flocculators was inversely related to WTP output: higher outputs require a minimum speed of 6 rpm, while lower outputs can use a maximum of 13 rpm. Analysis shows that setting all four hyperboloid flocculators to the same speed of 10 rpm achieves optimal conditions, enhancing flocculation
for this water source.

Comparative analyses from samples taken between November 26 and December 21, 2007, demonstrate that hyperboloid flocculators in train 4 outperformed paddle wheels in train 2 in terms of efficiency: total iron reduced by approximately 0.41 mg/l, total manganese by 0.10 mg/l, along with turbidity and color also seeing significant decreases. This results in longer runs at a higher filtering velocity and an annual savings of about 82% in power consumption when replacing paddle wheels with hyperboloid flocculators.

7. Conclusion

8. Literature

Droste, R. L., & Gehr, R. (2018). Theory and practice of water and wastewater treatment (2nd ed.). Hoboken, NJ: John Wiley & Sons.

Metcalf & Eddy Inc., Tchobanoglous, G., Burton, F. L., Tsuchihashi, R., & Stensel, H. D. (2014). Wastewater Engineering: Treatment and Resource Recovery (5th ed.). New York, NY: McGraw-Hill Education

MWH. (2012). Water treatment: Principles and design (3rd ed.). Hoboken, NJ: John Wiley & Sons.

Sawyer, C. N., McCarty, P. L., & Parkin, G. F. (2003). Chemistry for environmental engineering and science (5th ed.). New York, NY: McGraw-Hill Education.